1. Field of the Invention
The present invention relates to the field of etching substrates for use in integrated circuits, and more particularly to the field of anisotropic wet etching and the use of amorphous etch stop regions.
2. Discussion of Related Art
A problem found in etching substrates on which structures have been formed is microloading. Microloading is the influence that the differing densities of structures in different regions on a single substrate will have on the etch geometry in those different regions. Examples of different etch geometries in regions having different structure densities are illustrated in FIG. 1. Region 1 is a region having a relatively high density of structures 110 formed on the substrate 120. The structures 110 used for example in FIG. 1 are transistor gates 130 having sidewall spacers 140. In this example, the substrate is etched to form recesses that are subsequently backfilled with a doped material to form source/drain regions. Region 2 is a region having a relatively low density of structures 110 formed on the substrate 120. The etched areas 150, which may be source/drain regions for transistor gates 130, have different etch geometries in regions 1 and 2. The etched areas 150 in the relatively dense Region 1 undercut less area of the sidewall spacers and transistor gates than do the etched areas 150 in Region 2 and also tend to have less depth than the etched areas 150 in Region 2. For example, as illustrated in FIG. 1, the undercut areas 160 in Region 1 only undercut the sidewall spacers 140, but the undercut areas 170 in Region 2 undercut both the sidewall spacers 140 and the transistor gates 130. Microloading is a significant problem affecting the performance of integrated circuits because it results in the formation of devices on a substrate that have inconsistent structures as compared to other devices on the same substrate.
Microloading has been dealt with in the past by forming dummy structures on a substrate so that the density of structures on the substrate is equal everywhere on the substrate. Dummy structures are not ideal because they take up space on a substrate that may be put to better use and because large spaces between structures may be needed for specific device requirements.
Microloading has also been dealt with in the past by forming an etch stop within the substrate to control the depth of the etching. The prior art has formed an etch stop in the substrate by doping the substrate with extrinsic elements such as boron (B), phosphorous (P), and arsenic (As). The etch stop helps control the depth of an anisotropic wet etch. By using an etch stop with an anisotropic wet etch, both the depth of the area etched as well as the width (undercut) of the area etched may be controlled. The drawback to using elements such as boron, phosphorous, and arsenic is that they may diffuse from the etch stop area into regions where they may cause electrical interference with devices formed in or on the substrate.
A double spacer process has also been used to counter the effects of lateral undercutting caused by microloading. In this method the lateral undercut is controlled by first forming narrow sidewall spacers on either side of the gate electrodes. A hole is then etched with an anistropic dry etch and then filled with the material of interest. Another sidewall spacer is then formed and the substrate between the spacers is implanted with an extrinsic element. But, this method requires a number of steps and will not fully prevent the problem of inconsistent undercutting due to microloading and cannot be used when undercutting for structures such as source/drain tip extension regions.